1. Field of the Invention
This invention relates to vacuum variable capacitors.
2. Related Art
Vacuum variable capacitors are used in a variety of applications in a variety of industries. For example, vacuum variable capacitors are used in RF matching devices to vary capacitance as part of tuning the RF impedance.
FIG. 1 is a cross-sectional view of a previous vacuum variable capacitor 100. Each of two opposed capacitor plate structures 102 and 103 include a mounting plate 102b or 103b, respectively, having formed thereon a multiplicity of concentric cylindrical capacitor plates 102a or 103a, respectively. (In the following description, the reference numerals 102a and 103a can refer to one or multiple capacitor plates.) The capacitor plates 102a and 103a are formed on the mounting plates 102b and 103b, respectively, at locations and with a spacing such that, as shown in FIG. 1, the capacitor plate structures 102 and 103 can be positioned with respect to each other to cause capacitor plates 102a of the capacitor plate structure 102 to fit between adjacent capacitor plates 103a of the capacitor plate structure 103 (and vice versa) so that a desired spacing (xe2x80x9cgap distancexe2x80x9d ) between adjacent capacitor plates 102a and 103a results. The capacitor plate structures 102 and 103 are electrically connected to a voltage source so that the adjacent capacitor plates 102a and 103a act as a capacitor.
The capacitor plate structures 102 and 103 are positioned in a housing 101. The capacitor plate structure 103 is attached to the housing 101 so that the position of the capacitor plate structure 103 remains fixed with respect to the housing 101. As explained further below, the capacitor plate structure 102 is attached to the housing 101 so that the position of the capacitor plate structure 102 can move with respect to the housing 101.
An end of a hollow shaft 104 is attached to the, capacitor plate structure 102. A female threaded member 109 is attached to an end of the shaft 104 opposite the end attached to the capacitor plate structure 102. A male threaded member 105 is screwed into the threaded member 109. The threaded member 105 is attached to an adjustment, head 108 which is, in turn, attached to the housing 101 so that the adjustment head 108 and threaded member 105 are held in place with respect to the housing 101 along a longitudinal axis 110 of the vacuum variable capacitor 100. Rotating the adjustment head 108 causes the threaded member 105 to move into or out of the threaded member 109, causing corresponding motion of the threaded member 109, shaft 104 and capacitor plate structure 102 (including associated capacitor plates 102a) with respect to the housing 101 along the longitudinal axis 110. Since the position of the capacitor plate structure 103 with respect to the housing 101 is fixed, the adjustment head 108 can therefore be used to change the relative positions of the capacitor plates 102a and 103a, thereby adjusting the capacitance between the capacitor plates 102a and 103a, as known to those skilled in the art.
A bellows 106 surrounds the shaft 104. A bearing 107 enables the shaft 104 to rotate relative to the bellows 106 and the housing 101. The housing 101, bearing 107, bellows 106 and mounting plate 102b form a sealed enclosure, held at a vacuum pressure, within which the capacitor plates 102a and 103a are positioned. The bellows 106 expands and contracts as necessary to allow movement of the threaded member 109, bearing 107, shaft 104 and capacitor plate structure 102 along the longitudinal axis 110. The bellows 106 also provides an electrical connection from the capacitor plate structure 102 to complete the electrical circuit including the capacitor formed by the capacitor plates 102a and 103a. 
The above-described vacuum variable capacitor 100 is illustrative of previous vacuum variable capacitors. (Variations exist: for example, in another type of previous vacuum variable capacitor, the capacitor plates are formed as spirals, rather than as concentric cylinders as in the vacuum variable capacitor 100.) The vacuum variable capacitor 100 has a number of characteristics which can be undesirable.
The structure for effecting movement of the capacitor plate structure 102 is subject to mechanical friction which can cause adjustment of the capacitance to take an undesirably long time and/or require an undesirably large amount of power. Additionally, movement of the capacitor plate structure 102 may be opposed by a force due to the differential pressure between the vacuum pressure within the sealed enclosure and the atmospheric pressure outside of the sealed enclosure. The necessity of overcoming this force can further cause adjustment of the capacitance to take an undesirably long time and/or require an undesirably large amount of power. Illustratively; in the vacuum variable capacitor 100, capacitance can be adjusted at a rate of about 200 picofarads per second.
As indicated above, the bellows 106 provides an electrical connection from the capacitor plate structure 102 (i.e., from the capacitor represented by the capacitor plates 102a and 103a) to complete the electrical circuit of which the capacitor is part. However, the bellows 106 has associated therewith a parasitic inductance and resistance which degrades the performance of the capacitor.
The capacitor plate structures 102 and 103 are typically made by brazing the capacitor plates 102a or 103a on to the corresponding mounting plate 102b or 103b. The brazing process softens the material (typically copper) of which the capacitor plates 102a and 103a are made. This can make the capacitor plates 102a and 103a undesirably susceptible to deformation that degrades the performance of the vacuum variable capacitor 100 or renders the vacuum variable capacitor 100 unusable.
The size of the vacuum variable capacitor 100 along the longitudinal axis 110 may be larger than desired for some applications. (For convenience, the overall size of the vacuum variable capacitor 100 in this dimension is referred to herein as the xe2x80x9clengthxe2x80x9d of the vacuum variable capacitor 100.) For example, the presence of the bellows 106 adds to the length of the vacuum variable capacitor 100. The shape of the capacitor plates 102a and 103a can also affect the length of the vacuum variable capacitor 100, as explained in more detail below with respect to the description of the invention.
The housing 101 of the vacuum variable capacitor 100 is cylindrical. In previous vacuum variable capacitors, the housing has been made cylindrical to avoid stress concentrations that may otherwise occur at corners of a rectangular housing as a result of the differential pressure between the vacuum pressure within the housing and the atmospheric pressure outside of the housing. However, while, for the reason given above, the use of a cylindrical housing may be desirable if only the construction of the vacuum variable capacitor is considered, the use of a cylindrical housing may not be desirable from the standpoint of a system with which the vacuum variable capacitor is to be used, since a cylindrical shape may not be as space-efficient as a rectangular shape when the vacuum variable capacitor is integrated with other components of the system.
A vacuum variable capacitor according to the invention can include one or more characteristics that provide advantages over previous vacuum variable capacitors. In particular, a vacuum variable capacitor according to the invention can be constructed so as to enable the capacitance to be adjusted more easily, reduce parasitic electrical characteristics that degrade the performance of the vacuum variable capacitor, increase the strength of the capacitor plates of the vacuum variable capacitor, reduce the size of the vacuum variable capacitor, and/or make the shape of the vacuum variable capacitor more easily integrated into a system of which the vacuum variable capacitor is part.
A vacuum variable capacitor according to the invention includes: i) a first capacitor plate structure including one or more capacitive surfaces; ii) a second capacitor plate structure including one or more capacitive surfaces; iii) apparatus for forming a sealed enclosure; and iv) apparatus for moving the first capacitor plate structure and/or the second capacitor plate structure. The sealed enclosure is held at vacuum pressure. The first and second capacitor plate structures are enclosed within the sealed enclosure and are positioned with respect to each other so that corresponding capacitive surfaces of the first and second capacitor plate structures are spaced apart from each other. The movement apparatus is adapted to move the first capacitor plate structure and/or the second capacitor plate structure, such that the spacing between corresponding capacitive surfaces of the first and second capacitor plate structures changes, thereby changing the capacitance of the vacuum variable capacitor.
In one embodiment of the invention, no part of the movement apparatus moves in opposition to a force due to a pressure differential between the inside and the outside of the sealed enclosure. This can be accomplished by enclosing all moving components of the movement apparatus within the sealed enclosure. The movement apparatus can be embodied by a coil and magnet electromagnetically coupled to each other, one of the coil or magnet movably mounted inside the sealed enclosure and connected to one of the first or second capacitor plate structures that is movably mounted, the other of the magnet and coil mounted outside of the sealed enclosure. Since no part of the movement apparatus moves in opposition to a force due to a pressure differential between the inside and the outside of the sealed enclosure, it is easier to move the capacitor plate structure(s) in a vacuum variable capacitor according to this embodiment of the invention than it has been to move a capacitor plate structure in a previous vacuum variable capacitor, thus making it easier (i.e., requiring less time and/or power) to effect a change in capacitance in the vacuum variable capacitor according to the invention than in the previous vacuum variable capacitor.
In another embodiment of the invention, the movement apparatus includes a coil and a magnet electromagnetically coupled to the coil. One of the magnet and coil is movably mounted inside the sealed enclosure, while the other is mounted outside of the sealed enclosure. The movement apparatus further includes apparatus for connecting the one of the magnet and coil that is inside the sealed enclosure to one of the first or second capacitor plate structures that is movably mounted, thereby enabling movement of the magnet or coil to effect corresponding movement of the first or second capacitor plate structure. In a further embodiment, the vacuum variable capacitor includes an electrically conductive partition positioned within the sealed enclosure to separate the first and second capacitor plate structures from the one of the magnet and coil that is movably mounted inside the sealed enclosure. The use of a coil and magnet enables the movable capacitor plate structure to be driven without use of threaded members in order to translate rotary motion to linear motion and without need to provide a bellows or movable vacuum seal; thus, much of the mechanical friction that may otherwise be associated with movement of the capacitor plate structure (as in previous vacuum variable capacitors) is eliminated, thereby eliminating the need for overcoming such friction when moving the capacitor plate structure and enabling capacitance to be changed more easily (i.e., to require less time and/or power). Further, positioning of the movable magnet or coil inside the sealed enclosure can preclude the need to overcome a force due to a pressure differential between the inside and the outside of the sealed enclosure, further increasing the ease with which the movable capacitor plate structure can be moved and capacitance correspondingly changed. Positioning of the movable magnet or coil inside the sealed enclosure can also eliminate the need to use a bellows to provide a flexible seal that maintains the vacuum pressure within the sealed enclosure while transmitting motion from a mechanical driving apparatus outside the sealed enclosure to the movable capacitor plate structure inside the sealed enclosure. The elimination of the bellows can enable the length of the vacuum variable capacitor to be reduced as compared to previous vacuum variable capacitors.
In yet another embodiment of the invention, the vacuum variable capacitor further includes a third capacitor plate structure including one or more capacitive surfaces. The first and third capacitor plate structures are positioned with respect to each other so that the capacitive surfaces of the third capacitor plate structure are spaced apart from corresponding capacitive surfaces of the first capacitor plate structure. The movement apparatus is adapted to move the first capacitor plate structure, the second capacitor plate structure and/or the third capacitor plate structure, such that the spacing between corresponding capacitive surfaces of the first and second capacitor plate structures and/or the first and third capacitor plate structures changes, thereby changing the capacitance of the vacuum variable capacitor. The use of two capacitor plate structures (the second and third capacitor plate structures) on one side of a gap across which capacitance is established eliminates the need to use a bellows or other moving conductive path to provide an electrical connection from the capacitor plate structure (the first capacitor plate structure) on the other side of the gap to complete the electrical circuit including the capacitor formed by the capacitor plates of the capacitor plate structures, since the two capacitor plate structures on one side of the gap provide the necessary two electrical connections from the single capacitor plate structure on the other side of the gap. Thus, the parasitic inductance and resistance associated with the bellows is eliminated.
In still another embodiment of the invention, the sealed enclosure is formed in a housing. The first and/or second capacitor plate structures are movably mounted in the sealed enclosure to enable movement along an axis. The housing has a rectangular cross-sectional shape in at least one plane that is perpendicular to that axis. In a further embodiment, the at least one plane includes a plane including the edges of the housing which extend farthest from a central axis of the vacuum variable capacitor. In another further embodiment, the movement apparatus includes a coil and a magnet electromagnetically coupled to each other, wherein the magnet and coil are mounted inside and/or around the housing, and the part of the housing within or around which the magnet and coil are mounted has a circular cross-sectional shape in at least one plane that is perpendicular to the axis along which the first and/or second capacitor plate structure can move. The use of a housing having a rectangular cross-sectional shape can enable the vacuum variable capacitor to be integrated more compactly together with other components of a system with which the vacuum variable capacitor is to be used, and can, depending upon the shape of components within the housing, enable a smaller housing and/or smaller system housing to be used.
In another embodiment of the invention, each of the first and second capacitor plate structures includes a mounting plate and one or more capacitor plates formed thereon. Each capacitor plate has opposed capacitive surfaces that each lie in a plane (i.e., the capacitor plates are xe2x80x9cstraightxe2x80x9d). The use of capacitor plate structures with straight capacitor plates can be advantageous because such capacitor plate structures can be constructed using a relatively inexpensive form of electrical discharge machining (the use of which is advantageous for reasons discussed further below) known as wire electrical discharge machining.
In yet another embodiment of the invention, each of the first and second capacitor plate structures again includes a mounting plate and one or more capacitor plates formed thereon. However, in this embodiment, each capacitor plate has opposed capacitive surfaces that are each not perpendicular to the surface of the mounting plate on which the capacitor plate is mounted (i.e., the capacitive surfaces are xe2x80x9cangledxe2x80x9d). Since the capacitive surfaces of the capacitor plates are angled, moving the capacitor plate structures with respect to each other simultaneously varies both gap distance and capacitor plate overlap so that the capacitance varies more per unit of distance that the capacitor plate structures are moved relative to each other than would be the case if straight capacitive surfaces were used (as in previous vacuum variable capacitors). Consequently, capacitance can be varied more quickly. Additionally, a given amount of travel of the capacitor plate structures with respect to each other produces a larger change in capacitance, enabling a desired range of capacitance values to be achieved using capacitor plates having a smaller height, which, in turn, allows the length of the vacuum variable capacitor to be reduced.
In another embodiment of the invention, a capacitor plate structure is constructed by performing electrical discharge machining to form one or more capacitor plates on a mounting plate. In a further particular embodiment, wire electrical discharge machining is performed to form one or more capacitor plates having opposed capacitive surfaces that each lie in a plane (i.e., xe2x80x9cstraightxe2x80x9d capacitor plates). In an alternative further particular embodiment, probe electrical discharge machining is performed to form one or more capacitor plates having opposed capacitive surfaces that each do not lie in a plane (i.e., xe2x80x9ccurvedxe2x80x9d capacitor plates). The use of electrical discharge machining to form a capacitor plate structure is advantageous because electrical discharge machining does not soften the material of which the capacitor plates are made as much as brazing, thereby making the capacitor plates more resistant to deformation and reducing the likelihood of damage to the capacitor plates that can degrade the performance of the vacuum variable capacitor or render the vacuum variable capacitor unusable.